A lithium ion secondary battery (hereinafter, sometimes referred to as a lithium ion battery) is a secondary battery capable of attaining higher voltage and higher energy density compared to the conventional nickel-cadmium battery and nickel metal hydride battery, and since this enables the battery to become smaller in size and lighter in weight, it has been widely used for information-related mobile communication electronic equipment such as mobile phones and laptop personal computers. With regard to the lithium ion secondary battery, it is thought that the opportunity of being utilized for onboard use in which the battery is incorporated into electric vehicles, hybrid electric vehicles and the like or industrial use such as use in electric power tools will further increase in the future, and attaining further highly enhanced capacity and highly enhanced output has been eagerly desired.
The lithium ion secondary battery is composed of positive and negative electrodes having at least an active material capable of reversibly releasing and occluding lithium ions, a separator which is arranged in a container and separates the positive electrode from the negative electrode, and the container being charged with a non-aqueous electrolyte solution.
The positive electrode is prepared by applying an electrode agent containing an active material, a conductive additive and a binding agent to a metal foil current collector made of aluminum and the like and subjecting it to pressure forming. As the current positive electrode active material, a powder of composite oxides of lithium and a transition metal (hereinafter, sometimes referred to as lithium metal oxides) such as lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMnO2), or a ternary system material in which a portion of cobalt is substituted with nickel and manganese (LiMnxNiyCo1-x-yO2), and spinel type lithium manganate (LiMn2O4) has been used relatively frequently. Since these materials contain a so-called rare earth element, there is a problem in terms of cost and stable supply.
In recent years, olivine-based materials (phosphate-based materials) with a high level of safety have been attracting attention, and above all, lithium iron phosphate (LiFePO4) containing iron which is one of the abundant resources and is inexpensive has begun to be put into practical use. Moreover, lithium manganese phosphate (LiMnPO4) with a high level of output energy has also been attracting attention as a next-generation active material. Separately, metal oxides such as V2O5, metallic compounds such as TiS2, MoS2 and NbSe2, and the like have also been utilized.
Moreover, the negative electrode is prepared, as in the case of the positive electrode, by applying an electrode agent containing an active material, a conductive additive and a binder agent to a metal foil current collector made of copper and the like and subjecting it to pressure forming. Generally, examples of the active material for the negative electrode include lithium metal, lithium alloys such as a Li—Al alloy and Li—Sn, silicon compounds in which SiO, SiC, SiOC and the like are the basic constituent elements, conductive polymers prepared by doping lithium into polyacetylene, polypyrrole and the like, intercalation compounds prepared by allowing lithium ions to be incorporated into crystals, carbon materials such as natural graphite, artificial graphite and hard carbon, and the like.
In the ingredients of these positive and negative electrodes, the conductive additive plays a role in attaining an efficient conduction path from an active material to a current collector and is an essential constituent material to the electrode for a lithium ion battery.
However, when the content of the conductive additive is high, the battery capacity per weight of the electrode is lowered. Therefore, it is preferred that the amount of the conductive additive should be as small as possible, and there is a need for a conductive additive with high conductivity capable of securing the conductivity with a smaller amount thereof. Moreover, in recent years, there are many active materials that have not been put into practical use in spite of their high capacity since the conductivity thereof is low, for example, an olivine-based positive electrode active material, a solid solution-based active material and the like. In this respect, there is a need for a conductive additive with high conductivity.
Examples of the material conventionally used as a conductive additive include acetylene black, ketjen black and the like. Since these are inexpensive and have reasonable dispersibility, but have low crystallinity, their electrical conductivity is lower than that of graphite or the like and it is necessary to allow them to be incorporated in large quantities.
On the other hand, it is thought that since graphene has high conductivity, a shape of high aspect ratio and a large number of particles per unit weight, it has high potential as a conductive additive.
Graphene is one of two-dimensional crystals composed of carbon atoms and has been attracting attention since it was discovered in 2004. Graphene has excellent electrical, thermal, optical and mechanical properties and has a broad range of possible applications in the fields such as battery materials, energy storage materials, electronic devices and composite materials. If the potential of graphene is brought out, there is a possibility that a lithium ion battery electrode with high capacity and high output can be prepared when graphene is used as a conductive additive.
Examples of the preparation method of graphene include a mechanical exfoliation method, a chemical vapor deposition method, an epitaxial crystal growth method and an oxidation-reduction method. With regard to the former three methods (the mechanical exfoliation method, the chemical vapor deposition method and the epitaxial crystal growth method), the productivity is low and the mass production is difficult. On the other hand, since the oxidation-reduction method has the potential of mass production and further has the merit of being easy to perform chemical modification, it has been attracting attention.
The oxidation-reduction method presently proposed employs a heat reduction method, a method of using hydrazines and other organic compounds as a reducing agent or the like, and reduces a graphene oxide to prepare graphene.
Graphene is a type of nanocarbon and is usually less apt to be dispersed. Accordingly, for effectively utilizing graphene as a conductive additive for a lithium ion battery, the technique of dispersing graphene well is necessary.
In Non-Patent Document 1, an example of utilizing graphene for an electrode for a lithium ion battery is disclosed. In Non-Patent Document 2, an example of enhancing the dispersibility by adding a dispersing agent to graphene is disclosed. In Non-Patent Document 3 and Patent Document 1, examples of preparing graphene by reducing a graphene oxide with thiourea are disclosed.